MRAM, which is based on the integration of silicon CMOS (complementary metal on semiconductor) with magnetic tunnel junction (MTJ) technology, is a major emerging technology that is highly competitive with existing semiconductor memories such as SRAM, DRAM, and Flash. Furthermore, spin-transfer torque (STT) magnetization switching described by J. C. Slonczewski in “Current driven excitation of magnetic multilayers”, J. Magn. Magn. Mater. V 159, L1-L7 (1996), has led to the development of spintronic devices such as STT-MRAM on a gigabit scale.
Both field-MRAM and STT-MRAM have a MTJ element based on a tunneling magnetoresistance (TMR) effect wherein a stack of layers has a configuration in which two ferromagnetic (FM) layers are separated by a thin non-magnetic dielectric layer. One of the FM layers is a pinned layer having a magnetic moment that is fixed in a first direction while the other FM layer is called a free layer (FL) and has a magnetic moment that is free to rotate in a direction parallel (P state) or anti-parallel (AP state) to the first direction corresponding to a “0” or “1” magnetic state, respectively. Compared with conventional MRAM, STT-MRAM has an advantage in avoiding the half select problem and writing disturbance between adjacent cells. The spin-transfer effect arises from the spin dependent electron transport properties of ferromagnetic-spacer-ferromagnetic multilayers. When a spin-polarized current traverses a magnetic multilayer in a current perpendicular to plane (CPP) direction, the spin angular momentum of electrons incident on a FM layer interacts with magnetic moments of the FM layer near the interface between the FM layer and non-magnetic spacer. Through this interaction, the electrons transfer a portion of their angular momentum to the FL. As a result, spin-polarized current can switch the magnetization direction of the FL if the current density is sufficiently high, and if the dimensions of the multilayer are small.
P-MTJs are MTJ cells with perpendicular magnetic anisotropy (PMA) in the pinned layer and FL, and are the building blocks that enable STT-MRAM and other spintronic devices. Typically, there is a non-magnetic tunneling oxide layer called a tunnel barrier layer between the pinned layer and FL. When the FL has PMA, the critical current (iC) needed to switch the FL and p-MTJ from a P state to an AP state, or vice versa, is directly proportional to the perpendicular anisotropy field as indicated in equation (1) where e is the electron charge, α is a Gilbert damping constant, Ms is the FL saturation magnetization, h is the reduced Plank's constant, g is the gyromagnetic ratio, and Hkeff,⊥ is the out-of-plane anisotropy field of the magnetic region to switch, and V is the volume of the free layer:
                              i          c                =                              α            ⁢                                                  ⁢                          eMsVH                                                k                  eff                                ,                ⊥                                                          g            ⁢                                                  ⁢            ℏ                                              (        1        )            
The value Δ=kV/kBT is a measure of the thermal stability of the FL where kV is also known as Eb or the energy barrier between the P and AP magnetic states, kB is the Boltzmann constant and T is the temperature. Thermal stability is a function of the perpendicular anisotropy field as shown in equation (2):
                    Δ        =                                            M              s                        ⁢                          VH                                                k                  eff                                ,                ⊥                                                          2            ⁢                          k              B                        ⁢            T                                              (        2        )            
The perpendicular anisotropy field (Hk) of the FL is expressed in equation (3) as:
                              H                                    k              eff                        ,            ⊥                          =                                            -              4                        ⁢            π            ⁢                                                  ⁢                          M              s                                +                                    2              ⁢                              K                U                                  ⊥                                      ,                    S                                                                                                      M                s                            ⁢              d                                +                      H                          k              ,              ⊥                                                          (        3        )            where Ms is the saturation magnetization, d is the thickness of the free layer, Hk,⊥ is the crystalline anisotropy field in the perpendicular direction, and KU⊥,S is the surface perpendicular anisotropy of the top and bottom surfaces of the FL. Since the FL must be able to withstand 400° C. temperatures during annealing processes necessary for CMOS fabrication, this high temperature requirement has led to new p-MTJ designs wherein the FL has greater PMA. One approach to enhancing PMA in a FL is to form a metal oxide interface at top and bottom surfaces thereof. Thus, in addition to a first FL interface with the tunnel barrier layer, a second FL interface is formed with a so-called Hk enhancing layer to generate higher surface perpendicular anisotropy in equation (3).
Since the Hk enhancing layer is usually underoxidized to minimize RA in the p-MTJ cell, there is a tendency for metals or other species from an overlying capping layer or hard mask to migrate through vacant lattice sites in the Hk enhancing layer to the FL and degrade DRR. DRR is expressed as dR/R where dR is the difference in resistance between the P and AP states, and R is the resistance of the P state. Larger DRR means a higher read margin. Moreover, oxygen may migrate out of the Hk enhancing layer, and thereby reduce surface perpendicular anisotropy at the FL/Hk enhancing layer interface, which leads to lower FL thermal stability. Thus, an improved p-MTJ structure is needed to maintain Hk enhancing layer integrity such that FL thermal stability is maintained while providing DRR and RA values required for high magnetic performance in advanced memory designs wherein a critical dimension (FL width) is substantially less than 100 nm.